Ebook: The Design of High Performance Mechatronics

Preface

Motivation to the First Edition

A world without mechatronics is almost as unthinkable as a world without electric light. After its origin around the second world war the name mechatronics has become known for all kind of mechanical systems where mechanics and electronics are combined to achieve a certain function. The complexity of mechatronics ranges from a simple set of electronic controlled relay-switches to highly integrated precision motion systems. This proliferation of mechatronics has been accompanied by many books, which each have been written with a different scope in mind depending on the specific technological anchor point of the author(s) within this wide multidisciplinary field of engineering.

This book distinguishes itself from other books in several ways. First of all it is a combination of an industrial reference book and a university textbook, due to the mixed industrial and academic background of the authors. The industrial reference book part is based on extensive experience in designing the most sophisticated motion systems presently available, the stages of wafer scanners, which are used in the semiconductor industry. The academic part is based on advanced research on precision motion systems, including ultra precision metrology equipment with fast Scanning-Probe Microscopy and optical measurement systems with sub-nanometre accuracy. Closely related to the industrial background is the focus on high-precision positioning at very high velocity and acceleration levels. With this focus, the book does not include examples from other important application areas like robotics, machining centres and vehicle mechatronics, though the theory is also valid for those applications. All presented material is aimed at obtaining a maximum of control of all dynamic aspects of a motion system, which is the reason for the term “High Performance” in the title.

Another, more teaching related reason for writing this book next to all other books in the field is based on the observation that most students at the university are rather well trained in applying mathematical rules for solving purely mathematical problems, while they often have more difficulties in the application of these mathematics in the modelling of real mechatronic designs. The industrial need for well educated real engineers with both theoretical and practical skills, combined with a healthy critical attitude to the outcome of computer simulations, became a guiding motive to finish the tedious job of writing. The capability to swiftly switch between model and reality is one of the most important skills of a real multidisciplinary designer. This capability helps to quickly predict the approximate system behaviour in the concept phase of a design, where intuition and small calculations on the backside of an envelope are often more valuable than computer based detailed calculations by means of sophisticated modelling software. It is certainly true that these software tools are indispensable for further detailing and optimisation in the later phase of a design project but more attention is needed for basic engineering expert-knowledge to cover the concept-design phase where the most important design decisions are taken.

In view of these main motivations to write this book, it was also decided to focus uniquely on the hardware part of mechatronic systems. This means that the important field of embedded software is not presented even though software often serves as the actual implementation platform for modern control systems. The reason for this exclusion is the intended focus of this book on the prime functionality of a mechatronic system, without the interfaces to other systems and human operators. The logical sequence algorithm of the controller, together with the sampling delay, is more important for this prime functionality than the way how this algorithm is described in C-code.

When writing a book on mechatronics, the broad range of contributing disciplines forces a limitation in the depth to which the theory on each of these disciplines can be treated. Where necessary for the explanation of certain effects the presented material goes deeper, but other subjects are treated in such a way that an overall understanding is obtained without specialised in depth knowledge of all details.

Like the work of a mechatronic engineer as system designer in a team of specialists, this book is aimed to be a binding factor to the related specialised books, rather than one that makes these other sources of knowledge redundant.

It is our sincere wish that this book serves its purpose.

Robert Munnig Schmidt, Georg Schitter, Adrian Rankers and Jan van Eijk

January 2014

To the Second Edition

As many writers will agree, any book will contain errors in spite of a thorough check of every line. In this case most of these errors were typos and UK language issues, but gradually it became clear that also some unbalance existed in the chapters, mainly regarding the basic background material on mechanics and dynamics. The book was initially mainly intended for mechanical engineering students with BSc level knowledge. It appeared however that also students from other disciplines would follow the related courses and this made us decide to add some basic mechanics in this second edition. Further a good friend and specialist in active dynamics in the Netherlands, Adrian Rankers, made us aware of a real error in the modal analysis of the rotating body at the end of Chapter 3 and while investigating a solution, it was concluded that the part on modal analysis deserved a much deeper treatise in view of the frequent application in controlled mechatronic systems. Adrian was happy to provide the material from his PhD thesis [70] for inclusion in this book and he carefully reviewed the entire dynamics chapter. Also several remarks of students during lectures and examinations pointed clearly at some parts of the text that could result in a better understanding when written in a slightly different way. And last but not least several readers expressed their interest in literature citations for reference.

Summarising the following major changes are applied:

• Chapter 2 is renamed into “Applied Physics in Mechatronic Systems”. The mechanical laws of Newton and Lagrange are added, explaining coordinate systems and the methods to derive equations of motion from energy and acceleration. Several illustrations are added to explain the Fast Fourier Transform window and the Laplace plane while the Maxwell Equations are transferred from Chapter 5 to this chapter.

• Chapter 3 is extended with a large section on the theory on modal decomposition.

• Chapter 4 is extended with an introduction in discrete-time control.

• Relevant sources and subjects for further reading like PhD theses are cited in the text and grouped in a Bibliography section.

Finally the contributing authors expressed the wish to be more clear about their specific contribution. For this reason at each chapter the contributing authors are mentioned separately.

Contributions and Acknowledgements

Besides much material from our own experience, this book also includes material created by many other people.

Several university staff members and students have contributed to and reviewed the material. Some are cited in the text but even then it is impossible to mention all without forgetting some names. as example only the most important students who are not cited separately are mentioned here. The first is Ton de Boer, the MSc student who entered upon the impossible task to write the rough material that started this book as lecture notes by following the lectures on Mechatronic System Design. Johan Vogel and Oscar vd Ven, also from the Mechatronic System Design group at Delft University of Technology reviewed the first versions and helped with the physics and electromechanics chapters, while Markus Thier from the Automation and Control Institute at Vienna University of Technology helped with the section on digital motion control.

Our partners from industry deserve gratitude for their support, financially, in equipment or advice, by permission to use company illustrations or by reviewing the material. The three most important to mention are the Dutch high-tech company ASML and the metrology companies Heidenhain from Germany and Agilent Technologies from the United States.

We further thank all other companies and individuals who kindly gave permission to use their illustrations. Where appropriate these are separately mentioned at the related figures or cited in the bibliography.

It is true to say that this textbook is based on the knowledge of many others as laid down in books, patents and journal articles. Several are cited in the text but most are not, because their knowledge entered the public domain very long ago. Still it are these giants on whose shoulders we all stand

Cited with a slight variation to the words of Bernard de Chartres ≈1115.

and who deserve our gratitude. In that respect it is worthwhile to mention the increasing value of Wikipedia. Besides the possibility to quickly find certain physical and mathematical terms or derivations, it also provided information about small trivia like the date of birth or the full name of a famous scientist from the past.

Errata

The errata of the first and second edition are noted at a dedicated website: errata.rmsmechatronics.nl.

Summary

This book is intended for Bsc level students as an introduction to mechatronics, for Msc-level students who want to extend their knowledge on all aspects of advanced mechatronics and for engineers in the high-tech industry who want to learn more about adjacent specialisations. To accommodate this broad approach and define the application environment, the first and last chapter describe the waferscanners of ASML as these complex systems are symbolic for the high level of advancement that modern mechatronic systems have achieved.

The nine chapters are summarised as follows:

The introduction in Chapter 1 gives the context of mechatronics in the Dutch high-tech industry with the historical background, some general observations on the international differences in approach towards mechatronics and the close link with “Systems Engineering”. Subjects include the development of the optical Video Long Play (VLP) disk and the wafer stepper at Philips Research Laboratories. These developments have strongly determined the dominant foothold of high-precision mechatronic system design in the Netherlands and are exemplary for the specific photon-physics oriented approach in this country, quite different from the machining oriented approach in most other countries. The overview on systems engineering and design introduces some functional design and development methodologies that have proved to be crucial for the success of the high-tech industry. These methods are based on industrial practice where complex multidisciplinary designs have to be realised. Systems Engineering is a field closely related to mechatronics and the corresponding principles are used in structuring the design of a mechatronic system.

Chapter 2 on the applied physics in mechatronic systems is the first of a series of chapters on the theory that is applied in controlled motion systems. After an introduction to some relevant items from the mechanical domain, like coordinate systems and the physical laws on force and motion, the chapter introduces the theory on electricity and magnetism, essential element in a mechatronic positioning system. This is followed by a section on signal theory and wave propagation. This chapter explains the reason why the properties of mechatronics are so often described in the frequency domain next to the more mechanical oriented time-related step and impulse responses. The chapter also introduces different graphical representations of frequency responses, which are used in several chapters of this book.

The hard-core of a mechatronic system is still the mechanical structure that represents the real hardware, which has to be fully mastered when positioning objects in a controlled way. In most cases, the dynamic properties of this structure determine the achievable control performance. Expert knowledge of this field is a prerequisite for a mechatronic designer. For that reason Chapter 3 deals with these dynamics of motion systems and mainly concentrates on the vibrational properties of standard mechanical elements consisting of a multitude of springs, bodies and dampers. It includes a more in depth treatise of modal decomposition, a method to describe the dynamic response to external forces by means of individual vibration modes, which allow to optimise the structural dynamics for controlled motion.

Directly related to the mechanical dynamics is the important field of motion control in Chapter 4. This chapter concentrates on a thorough understanding of the working principle and tuning of the still widely used PID controllers. The practice of loop-shaping for optimising a feedback controller is introduced as it is widely used. Also an introduction is given in state-space control with direct pole placement as this method plays an increasing role in the design of mechatronic systems. A strong emphasis is put on the physical aspects of control. It is shown that feedback control adds virtual elements from the mechanical domain to the system, like springs and dampers together with new elements like an integrator and observer.

Electromechanic actuators and analogue electronics are two closely related hardware components of a mechatronic system. Their interaction is increasingly underestimated by system designers, because of two reasons. Firstly the field is controlled by experts in physics and electronics. These specialists have a fundamentally different more abstract frame of mind than the mostly concrete-mechanical visually oriented system designers. The second reason for underestimating these related fields is caused by the overwhelming amount of electronics, motors and actuators, which are all around us, giving rise to the idea that their principle is simple and mastered by many. This idea is a dangerous delusion as the difficulty in electronics is related to its dynamic analogue behaviour and unfortunately the number of people that master analogue electronics is rather decreasing than increasing. It is the analogue side of electronics, dealing with measurement and actuation, that needs most of the attention of the mechatronic designer.

With this purpose in mind, Chapter 5 first presents linear electromechanic actuators. This chapter focuses on electromagnetic actuators while also piezoelectric actuators are presented as these are increasingly applied in precision mechatronic systems. This chapter will help in the selection process of actuation systems and creates a knowledge base for further study on the subject. Also the relation with power-amplifier constraints, which are presented in the following chapter, is made clear.

Chapter 6 deals with analogue electronics for measurement and power and starts at a very basic level with passive components because most mechanical engineering students have hardly any knowledge about electronics. The introduction of the active components leads to their application in the basic design of the operational amplifier, the most universal and widely used analogue electronic building block. The last section in this large chapter gives an overview of the basic design of power amplifiers, which act as the interface between the controller and the actuators.

Optics has become a main driver of mechatronic advancement in the past decades. Firstly it is an application area where mechatronics are used to control and correct optical properties of imaging systems and other instrumentation. Secondly, optics are used to determine distances in a plurality of sensors, which enables us to create measurement systems with extreme precision. For these reasons Chapter 7 gives an introduction to optics from the perspective of a mechatronic designer. Starting with basic physics on optics with sources and the duality of light, an overview of geometrical and physical optics is presented including limiting factors for the performance of imaging systems. The chapter concludes with an introduction on adaptive optics.

Chapter 8 presents the basic principles of sensors for force and dynamic position measurements based on several physical principles including strain-, inductive-, capacitive- and optical sensors. The theory in this chapter will enable the first selection of suitable sensors when designing a mechatronic system. Laser interferometry and encoders will also be presented as these are most frequently applied in high precision mechatronic systems. Even though metrology in general will be shortly touched, the chapter concentrates on measurement for control. For this reason also the principle of dynamic error budgeting is included, a statistical method to determine the total error in a dynamic precision system from contributions of different error sources.

As closure of the book Chapter 9 presents the mechatronic design for precision positioning in waferscanners where all theory is applied to its most extreme level. This chapter includes the basic design of positioning stages, the need for and active control of vibration isolation, and the motion control approach to achieve a position accuracy of less them a nanometre at speeds of more than 1 m/s and accelerations of more than 30 m/s².